We propose to determine the structural and molecular mechanisms within yeast which control and execute reorganizations of subcellular components throughout the cell division cycle. (1) Conditional mutants of the division cycle of Saccharomyces cerevisiae will be examined by serial section electron microscopy to characterize the ultrastructural phenotype of these various specific genetic defects and the manner in which they perturb the overall division process, both in mitosis and in meiosis. (2) Proposed control of integration between nuclear and cytoplasmic events by the duplication of spindle plaques will be tested in the formation and first budding of zygotes. (3) Yeast will be fractionated in order to purify and characterize biochemically organelles playing specific roles in the division process. The synthesis, assembly, and activity of spindle plaques, microtubules, and microfibrils from mutant and synchronous wild-type strains will be correlated with factors which control the cell division cycle.